We have determined the atomic coordinates of yeast phenylalanyl tRNA in the monoclinic crystal form from an independent analysis from a Kendrew skeletal model built into a 3 A MIR map. Analysis of the molecular conformation has revealed that (a) the majority of the nucleotide moieties exhibit conformations preferred for the nucleotides themselves, (b) the internucleotide P-O ester bonds afford the primary source of flexibility for folding of the polynucleotide chain and (c) the sugar pucker and backbone C(4')-C(5') torsions provide the secondary source of flexibility. In continuation of this work data out to 2.43 A have been collected and the atomic coordinates are being subjected to refinement studies. Comparative studies of our structure with the MRC (Cambridge, U.K.) and MIT structures have shown similarities in the overall folding but important differences in the detailed stereochemistries of the nucleotides and phosphodiesters. An assorted number of crystalline complexes of dye-, drug-, mutagen- and carcinogen-tRNA have been made and their mode of interactions are being currently investigated by difference Fourier techniques. The above studies are expected to provide for the first time the association of dye and other molecules with a nucleic acid at the molecular level. BIBLIOGRAPHIC REFERENCES: Rubin, J., Brennan, T., Stout, C. D., Mizuno, H., Mallikarjunan, M., McMullan, R. K., Ichikawa, T., Rao, S. T. and Sundaralingam, M., "X-ray Diffraction Studies of Yeast Phenylalanine Transfer RNA: I. Isomorphous Heavy Atom Derivatives at 5.5 A Resolution." In Structure and Conformation of Nucleic Acids and Protein-Nucleic Acid Interactions. The Fourth Harry Steenbock Symposium, Madison, Wisconsin, June 16-19, 1975, University Park Press: Baltimore, Editors: M. Sundaralingam and S. T. Rao, pp. 25-37 (1975).